BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to new methods for producing aromatics. Specifically, the invention relates to catalytic . processes by which aromatic streams, containing nonaromatic impurities having boiling points in the same temperature range as the aromatics, are purified. Most particularly, the invention describes nonextractive catalytic processes for producing high purity aromatics.

Description of the Related Art

In the production of petrochemicals, high purity aromatic streams substantially free of nonaromatic compounds are needed. Suitable aromatic feeds for petrochemical production include benzene, toluene, and C _β aromatics. Typically, these aromatics are produced in admixtures with other components during catalytic naphtha reforming or as by-products of ethylene manufacture. For example, depending on the process variables and feed composition, the liquid product produced from naphtha reforming, referred to as reformate, contains a mixture of hydrocarbon species including aromatics such as benzene, toluene and xylene, commonly referred to together as BTX.

Once the aromatic compounds have been separated from close-boiling nonaromatic compounds, high purity

benzene, toluene and C ₈ aromatic streams may be recovered by distillation. These streams are then used in the manufacture of petrochemical intermediates such as ethylbenzene, styrene, cyclohexane, cumene, phenol, p- xylene, m-xylene, o-xylene and p-methylstyrene. Recovery of pure aromatic streams from a reformate or pyrolysis gasoline by distillation is not commercially practical due to the scaling requirements for the column or reflux ratio. Consequently, separation techniques that rely on differences in boiling point are impracticable for separating close- boiling nonaromatics, such as dimethyl cyclopentanes and cyclohexane, from the reformate or pyrolysis gasoline to yield relatively pure aromatic products. Further, while distillation can achieve at least gross separation of some nonaromatics from aromatics based on boiling points, the product may still contain a mixture of aromatics with one or more nonaromatic species having about the same boiling point. On a practical level, if the boiling point difference between the nonaromatic and aromatic species is less than about 10 ^* F, the distillate will contain a mixture of the species. Effective separation may be achieved when the boiling point difference is at least about 25 ^* F. Alternatively, separation may be achieved by resort to extractive methods using solvents that have a high relative affinity for aromatics compared to nonaromatics. Examples of extractive processes commonly used in industry are liquid-liquid extraction or extractive distillation, such as UOP\'s UDEX™ and SULFOLANE™ processes.

U.S. Patent Nos. 3,808,284 and 3,812,197 illustrate the use of distillation and solvent extraction methods in conjunction with catalytic

reforming processes.

Catalysts may be used to selectively remove nonaromatics from the reformate. For example, U.S. Patent No. 3,849,290 describes a multi-step process to upgrade the octane rating of a naphtha gasoline blending stock. In a first step, the stock is reformed over a nonacidic platinum-type catalyst, producing a reformate containing aromatics and paraffins. The reformate is then contacted under mild hydrocracking conditions with an intermediate pore zeolite to selectively crack high boiling, low octane paraffins, e.g., C ₇ +. The effluent from this hydrocracking step is contacted with a small pore catalyst to selectively hydrocrack low boiling, low octane C ₆ - paraffins. This three step, three catalyst processing sequence yields a high octane product after the preferential removal of low octane species, but the product purity is insufficiently high for petrochemical applications.

U.S. Patent No. 4,795,550 describes a low temperature catalytic process for removing olefinic impurities, but not paraffins or naphthenes, from an aromatic stream having a bromine index between 50 and 2000.

U.S. Patent No. 4,150,061 describes a process whereby a fractionated pyrolysis gasoline comprising toluene, xylenes, ethylbenzenes, C ₇ - C ₁₀ paraffins, olefins and naphthenes are selectively hydrodealkylated and transalkylated to give ethylbenzene-lean xylenes and benzene in the presence of a catalyst comprising a tungsten/molybdenum component ( 0 ₃ -Mo0 ₃ ) and an acidic component of 60 wt% mordenite and 40 wt% catalytically active alumina. The product is then distilled to provide benzene and xylene streams of unknown purity and a toluene stream for recycle.

U.S. Patent No. 4,861,932 describes a process for producing gasoline blending stocks in which nonaromatic C ₂ -C ₁₂ paraffins are converted to a mixture of higher octane aromatics and alkylaromatics by first contacting the paraffins with a noble metal/low acidity catalyst. The effluent is then contacted with an acidic catalyst based on a zeolite such as ZSM-5 with a metal such as gallium (Ga) . Although the Ga/ZSM-5 catalyst is known to have a high aromatic selectivity, the product purity is insufficiently high for petrochemical applications.

Improvements are still needed to develop catalytic processes that facilitate cost-effective production of high purity aromatic streams such as BTX with minimal contamination by nonaromatics having boiling points in the aromatics range.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide new catalytic processes for obtaining aromatic products of high purity. An additional aspect of the present invention is to provide a catalytic process by which nonaromatic components of an aromatic stream, boiling in the aromatic range, are converted in a single catalytic step to species that can be easily separated from the aromatics without resort to extractive techniques.

It has now been discovered that an aromatic feedstock containing close-boiling nonaromatics may be purified in a single catalytic step to yield high purity aromatic chemicals. Specifically, the aromatic feedstock is contacted with an acidic catalyst to yield a liquid product having nonaromatic impurities with boiling points substantially outside the boiling point

range for the aromatics. High purity or chemical grade aromatics may then be recovered from the product by separation, e.g. , simple distillation, at considerably less expense than in current commercial practices that use extractive methods.

In an additional embodiment, high purity benzene may be produced using a process in which nonaromatic impurities boiling in the same temperature range as benzene may be selectively eliminated by use of an acidic zeolite catalyst under elevated temperature conditions followed by distillation.

In another embodiment of the invention, high purity benzene, toluene or C ₈ aromatics are produced in a nonextractive process comprising reforming a naphtha stream over a nonacidic catalyst, reacting all or a portion of the reformate over an acidic catalyst and recovering .a high purity aromatic product by distillation. In a further embodiment of the invention, the reformate is distilled to obtain a benzene-, toluene-, or xylene-rich fraction prior to reaction over the acidic catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principal aim of the present invention is to produce high purity aromatic streams by selectively removing nonaromatic impurities having boiling points in the benzene, toluene and xylene (BTX) boiling range from aromatic streams by nonextractive techniques. This boiling range generally covers temperatures between about 140\'F and 350 ^* F (about 60 ^* C and 180 ^* C). These nonaromatics may be referred to herein as close-boiling nonaromatics.

The process in general involves cracking the nonaromatic impurities boiling in the aromatics range by

their selective reaction over an acidic catalyst. The close-boiling nonaromatics are converted to light paraffins and olefins boiling substantially outside the BTX range. In some instances, the light paraffins and olefins may form additional aromatics by alkylation or aromatization.

By "nonextractive", it is meant that separation of hydrocarbon species is achieved on the basis of differences in boiling point as opposed to reliance on a solvent. As a result of these nonextractive techniques, the purity of the aromatics produced is preferably at least about 95 wt%, more preferably at least about 99.0 wt%, even more preferably at 99.5 wt% and most preferably at least about 99.8 wt%. These aromatic products may be referred to herein as substantially free of nonaromatic impurities.

Catalyst acidity is essential to the nonextractive conversion step. As is known in the art, one measure of catalyst acidity is n-hexane cracking activity. For example, according to the "alpha" test, α represents the relative n-hexane cracking activity of a catalyst compared with a standard catalyst. The test is described more fully in U.S. Patent No. 3,354,078, the Journal of Catalysis/ vol. 6, p. 522-529, (Aug. 1965) and the Journal of Catalysis. Vol. 61, p. 395 (1980) .

According to the test, a catalyst is "acidic" if a is greater than about 10, or more preferably, is greater than about 50. Highly acidic catalysts have α values greater than about 100. Acidic catalysts that are useful in the present invention are characterized by this definition of acidity. In particular, useful acidic catalysts may be based on molecular sieves such as ZSM-5, beta-zeolite, X-zeolite, Y-zeolite, mordenite,

ZSM-11, SSZ-23 (described in U.S. Patent No. 4,902,844), SSZ-25 (described in U.S. Patent No. 4,826,667) and SSZ-26 (described in U.S. Patent No. 4,910,006). All of these patents are incorporated by reference. Due to their resistance to fouling, intermediate pore molecular sieves are preferred as the basis for acidic catalysts used processes in which the aromatic feed is in a gaseous phase. ZSM-5 is particularly preferred. For processes in which the aromatic feed is in a liquid state, acidic catalysts based on large pore molecular sieves, such as X-zeolite, Y-zeolite, mordenite, beta-zeolite and SSZ-26, are preferred.

The sieves are preferably bound with any of a variety of well-known inorganic oxide binders. Appropriate binders include inorganic compositions with which the molecular sieve can be combined, dispersed or otherwise intimately admixed. Preferred oxide binders include alumina, silica, naturally occurring and conventionally processed clays, for example, bentonite, kaolin, sepiolite, attapulgite and halloysite.

The selective conversion of nonaromatics in an aromatics-rich stream may be performed in either a liquid or gaseous state. Also, under appropriate conditions, a suitable feed may be provided where liquid and gas phases are in equilibrium.

In general, reaction conditions should be such as to promote the preferential catalytic conversion of the nonaromatics. For a liquid phase process, the process conditions, such as pressure, should be such that at least some portion of the feed remains in the liquid state. For example, the pressure should be between about 150-1000 psig. The preferred temperature is between about 500 ^* F to about 700 ^* F (260 ^* .C - 371"C).

The feed weight hourly space velocity may vary from 0.1 to 100, but is typically between 0.5 and 10. The presence of a liquid phase serves to wash the catalyst and keep it active. Alternatively, for a gas phase process, in general, the process conditions should be such that the feed remains gaseous. For example, the temperature may be relatively higher and the pressure may be relatively lower than for a liquid phase feed. Suitable reaction temperatures are between about 600 ^* F and 1200 ^* F (204 ^* C -

1168\'C), more preferably between about 700 ^* F and 1100 ^* F

(371 ^* C - 593 ^* C), and most preferably between about 950\'F and 1100\'F (482\'C - 593 ^* C). Pressures can be between atmospheric and 1000 psig or higher, but between 50 psig to 600 psig is preferred. In addition, it is preferred that the gas phase reaction be carried out in the presence of hydrogen to inhibit fouling. The molar ratio of hydrogen to hydrocarbon feed is typically between 0.5 and 5.0. The Examples that follow illustrate more specifically suitable operating parameters.

The processes of the present invention may be used to make high purity aromatics streams from a variety of feeds. Typical feeds include reformates, pyrolysis gasolines, and fractions and mixtures thereof. These feeds should contain at least about 70 wt% aromatics, preferably at least about 80 wt% and more preferably at least about 90 wt%, in order to minimize production of light gases and enhance both the cost- effectiveness of the processes of the present invention and their suitability for use with the nonextractive processes described herein.

In a preferred embodiment, a reformate obtained by reforming a naphtha feed is purified by reaction over

an acidic catalyst. Such a feed could be a light naphtha feed, for example, one rich in C ₆ and/or C ₇ components, reformed over any of a variety of conventional reforming catalysts to produce a product stream containing aromatics and close-boiling nonaromatics. Exemplary reforming process conditions include: feed rate of 0.1-10 WHSV, pressures between about 40 psig and 100 psig, temperatures between about 800\'F and 1100\'F, and a hydrogen:feed molar ratio of between about 0.1-10.

A nonacidic catalyst may be used to reform the naphtha feed and increase its aromatics content. Accordingly, α values should be less than 10 and preferably less than 0.1. In fact, strong catalyst acidity in aromatics generation is undesirable because it promotes cracking that in turn results in lower aromatic selectivity. To reduce acidity, the catalyst may contain an alkali metal and/or an alkaline earth metal. The alkali or alkaline earth metals are preferably incorporated into the catalysts during or after synthesis according to conventional methods. In addition, at least 90% of the acid sites are desirably neutralized by introduction of the metals, more preferably at least 95%, most preferably 100%. In addition, the catalyst for aromatics generation may be based on alumina or molecular sieves, such as L-zeolite or silicalite, with an inorganic binder. Preferred catalysts for reforming include catalysts comprising platinum on nonacidic forms of beta-zeolite, ZSM-5, silicalite and L-zeolite. Other well-known reforming catalysts typically contain a catalytic metal such as platinum disposed on any of a plethora of natural and man-made crystalline aluminosilicates. Metallic promoters such as Group VIII

metals rhenium and indium also may be included, as can other promoter metals such as tin and germanium.

Examples of methods of manufacture of ZSM-5 and particularly the ZSM-5 having high silica-alumina (Si0 ₂ :Al ₂ 0 ₃ ) molar ratio, sometimes referred to as silicalite are shown in: Dwyer, et al., U.S. Patent No. 3,941,871, issued March 2, 1976 and U.S. Patent No. 4,441,991, issued April 10, 1984; and Derouane, et al., EPO Application No. 186,479, published February 7, 1986, all of which are incorporated by reference in their entireties. Examples of the preparation of nonacidic platinum on silicalite or L-zeolite catalysts may be found in U.S. Patent Nos. 4,830,732 and 5,073,250.

Low sulfur feeds to the aromatics generation step may be particularly attractive in order to avoid poisoning the reforming catalyst. In the case of L- zeolite, it is preferable that the feed to the reformer has sulfur content less than 50 ppbw, and more preferably less than 5 ppbw. As a specific example of the employment of such an aromatics generation procedure with the process of the present invention, valuable C ₆ -C ₈ aromatics may be produced from relatively inexpensive hydrocarbon feeds such as pyrolysis gasolines and paraffinic naphthas. The products are streams containing ethylbenzene, benzene, toluene, and the three xylene isomers. The aromatics generation procedure converts such hydrocarbon feedstocks into very clean aromatic mixtures that, however, include close-boiling nonaromatic impurities. The process according to the present invention then converts the close-boiling nonaromatic impurities to yield a product that in turn may be distilled to yield high purity aromatic streams. Thus, the process uses two separate catalysts: a nonacidic catalyst.to generate

aromatics and an acidic catalyst to purify the aromatics generated.

More particularly, such a process could involve passing an inexpensive hydrocarbon feed over a nonacidic catalyst such as platinum on silicalite (alumina content between about 200-2000 ppm) . Such a catalyst is especially effective at aromatizing the feed, but, as described above, produces a complex mixture containing nonaromatics that are difficult to separate from the desired aromatics. This selectivity is valuable in the production of petrochemicals because of the high benzene content (about 70-75 wt%) product.

Next, as described in connection with other aspects of the present invention, an acidic catalyst cleans up the mixture by converting the close-boiling nonaromatic components into both lighter and heavier components that can be easily separated from the aromatics, for example, by distillation. The impure distillate contains C ₃ -C ₄ and C _β + hydrocarbons, the latter of which can be recycled for further aromatics generation. This beneficial combination of two different catalysts serves to facilitate the production of high purity aromatics such as BTX. The following example illustrates a process according to this embodiment.

EXAMPLE 1 A light reformate stream was aromatized over a nonacidic platinum on silicalite (alumina content less than 2000 ppm) catalyst to yield a product containing about 73 wt% total BTX and about 22 wt% of other C ₆ + hydrocarbons. The aromatics-enriched product stream (the liquid feed referred to in Table A) was contacted with an acidic Ga/HZSM-5 catalyst at 707\'F (375 ^* C) in

the presence of added hydrogen. In a second run, the liquid feed was contacted with the same acidic catalyst at 842\'F (450 ^* C) without hydrogen. Other process conditions are provided in Table A. As illustrated in Table A, the percentage of

BTX relative to the total C ₆ -C _β content of the liquid feeds and products increased from 76.6 wt% in the feed to about 94 wt% and about 98.9 wt% in the respective aromatics-enriched liquid product streams after treatment according to the method of the present invention. High purity aromatics can be easily separated from the product streams by distillation.

In an alternative embodiment of the present invention, nonaromatic impurities boiling in the same temperature range as benzene may be eliminated from a benzene stream by their selective reaction in the gas phase over an acidic zeolite catalyst under elevated temperature conditions. For example, at between 600 ^* F -

1200 ^* F (316 ^* C - 649 ^* C), C _s and heavier paraffins, olefins, and naphthenes crack to form light paraffins and olefins. The olefins alkylate benzene and other aromatics, leaving an aromatics fraction that is substantially free of close-boiling nonaromatics. Upon separation, aromatic purity levels of greater than about 95 wt% can be achieved.

High purity benzene may be manufactured by such a process that includes: reforming a predominantly C ₆ naphtha feed to yield greater than 70 wt% aromatics, separating a light fraction from a benzene and heavier fraction by distillation, passing the benzene and heavier fraction over an acidic HZSM-5 at 800 ^* F - 1100\'F (427"C - 593\'C) and recovering a highly pure benzene fraction by distillation. This process eliminates the need for expensive liquid extraction or extractive

distillation processes.

This embodiment of the present invention is further illustrated by Examples 2 and 3.

EXAMPLES 2 and 3

Two hexane-riσh (about 95 wt% hexane isomers) naphtha feedstreams were reformed in the presence of hydrogen over a nonacidic platinum on L-zeolite catalyst. Liquid reformate products were collected containing about 75 wt% benzene. The reformates were distilled to obtain 93 wt% and 99 wt% benzene fractions, respectively. The feeds to the acidic conversion step were these benzene fractions derived from each reformate. The fractions were vaporized and mixed with hydrogen and the combined streams were passed over acidic HZSM-5 catalysts at 150 psig, 1000\'F (538\'C), 4:1 hydrogen to hydrocarbon molar ratio, 5.7 HSV of liquid feed.

Tables B and C show that substantially all nonaromatic impurities in the feeds to the acidic conversion step were converted to C ₁ -C ₄ paraffins and C ₇ + aromatics. The aromatic products were recovered by condensation, while the light paraffins were carried off in the hydrogen stream product. Further distillation of the aromatics fraction would be expected to yield better than a 99.9 wt% purity benzene and a C ₇ + aromatics stream suitable for gasoline blending.

In another embodiment of the present invention, high purity benzene and xylenes can be made from impure toluene streams by using acidic catalysts with high cracking activity, such as beta-zeolite or HZSM-5. Under elevated toluene disproportionation conditions, up to about 10 wt% of nonaromatic impurities in the toluene feedstream are converted to light ends, enabling the recovery, for example, by distillation, of benzene, toluene and C ₈ aromatic fractions that are between 99.5 -

99.9 wt% pure. Thus, a toluene portion of a reformate or pyrolysis gasoline stream can be directed to a reactor where a process according to the present invention is carried out, bypassing an extraction, e.g., UDEX™, plant, and freeing up capacity for benzene production. The impure toluene stream, in either liquid or gaseous phase, can be reacted over the acidic catalyst to ultimately yield high purity aromatic streams. The present invention according to this embodiment is illustrated in Examples 4 and 5.

EXAMPLE 4

Debutanized reformate was distilled to obtain light and heavy fractions. The heavy reformate was further distilled to a 30% cut point. The overhead product of the second distillation comprising about 92 wt% toluene was vaporized, blended with hydrogen, and passed through a tubular fixed-bed reactor charged with an acidic ZSM-5 catalyst. The reaction was carried out at 1000\'F (538 ^* C), 150 psig, and 5.7 liquid toluene feed weight hourly space velocity (WHSV) . The hydrogen: toluene molar ratio was about three-to-one.

Feed and product analyses were obtained by

capillary gas-liquid chromatography after nine hours on stream, as shown in Table D below. The gas chromatograph was equipped with a flame ionization detector and a polar column that eluted nonaromatics before aromatics.

The analyses show that the toluene disproportionated to make benzene and xylenes, while nonaromatic impurities in the same boiling range were substantially eliminated by cracking to form light ends. The product can be distilled to obtain high purity aromatics.

EXAMPLE 5 Toluene and C ₇ nonaromatics were blended in a 92:8 weight ratio. The resultant liquid-phase mixture was reacted over an acidic beta zeolite catalyst in an up-flow, fixed-bed reactor. The reaction conditions were: liquid feed hourly space velocity (LHSV) of 0.5, temperature of 600"F (316\'C), and pressure of 600 psig. The feed and product streams were analyzed using a chromatograph equipped with a flame ionization detector and a nonpolar capillary column that eluted components by boiling point. Table E presents the results.

Table E

Retention Components Time, Minutes

Again, toluene was disproportionated to make benzene and xylenes, while close-boiling nonaromatics were simultaneously removed by cracking. Some of the resulting light ends alkylated benzene to form ethylbenzene and heavy aromatics that are valuable as gasoline blending components. The high purity benzene and xylenes may be separated by simple distillation.

As can be seen from the Examples, the invention described herein utilizes catalytic processes to produce high purity aromatics. In particular, the processes described herein provide effective and direct means to obtaining high purity aromatics such as benzene and C ₈ aromatics. Close-boiling nonaromatics are eliminated without resort to extractive techniques because of their preferential conversion by an acidic catalyst to nonaromatics that are easily separable from the aromatics on the basis of boiling point differences.

The versatility of the process is demonstrated in its combination with other processes such as reforming to produce high purity aromatic streams. Further, in view of these combinations, a wide variety of hydrocarbon feeds can be used, from impure aromatic streams to paraffinic feedstocks and the feeds may be in either liquid or gas form. Further, the process of the present invention contemplates the treatment of nonaromatic stocks that can be aromatized before the acidic purification step as described herein. Process conditions may be selected from wide temperature (500 ^* F - 1200\'F) and pressure ranges (0 - 1000 psig) based on the nature of the feed, catalyst and economic considerations.

Industrial Applicability

The present invention is useful for its ability to produce high purity aromatics, such as benzene and xylenes, from aromatic streams containing nonaromatic impurities boiling in the same temperature range as the aromatics. The purified aromatics can be used as petrochemical feedstocks.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.